This invention relates to a programmable controller system formed by connecting a programmable controller (hereinafter referred to as PLC) and a personal computer functioning as a control program development aid device through a communication means and in particular to such a PLC system having a user certification function.
Such PLC systems formed by connecting a PLC and a personal computer functioning as a control program development aid device through a communication means have been known. FIG. 15 shows an example of such a system comprising a PLC 2 and a personal computer 1a functioning as a control program development aid device 1, connected together by a communication means such as RS232C. In FIG. 15, A1 indicates a password memory area and A2 indicates a user program memory area.
The control program development aid device 1 is adapted to carry out communications with the PLC 2 through the communication means such as RS232C and to write and read a program into and from the user program memory area A2 through this communication means. The control program development aid device 1 is further adapted to monitor data in the memory area for setting the operations of the PLC 2 and the data in the memory area for storing input data from and output data to any target device.
The user certification function is incorporated in this PLC system. Japanese Patent Publication Tokkai 2003-162507, for example, has disclosed a PLC system of this kind having a user certification system. Explained more in detail, the password memory area A1 and the user program memory area A2 are provided to the PLC 2. When a communication command for reading out a program is issued from the control program development aid device 1 to the PLC 2, if any password data are written in the password memory area A1 in the PLC 2, the PLC 2 returns to the control program development aid device 1 a respond code which indicates that it is not possible to read out the program.
Both the password setting function and the password releasing function are incorporated to the control program development aid device 1. As the password setting function is activated, password data are written into the password memory area A1 of the PLC 2. As the password releasing function is activated, password data are read out from the password memory area A1 of the PLC 2 to the control program development aid device 1. At this moment, if the control program development aid device 1 ascertains that the password data inputted for releasing the password setting condition and the password data read out from the PLC 2 match, initialization of the password memory area A1 on the PLC 2 (the condition in which the password setting condition is released) becomes possible.
Problems as will be explained below are being pointed out, however, with PLC systems provided with such a prior art user certification function (the function of judging whether it is a certified user or not by using a password). Firstly, since the user certification method is a common method (such that it is written in a manual accessible to any user), any third party can pretend to be a normal user, say, by analyzing the mechanism of the user certification and repeating the input of password by trial and error. Secondly, there is a likely scenario that a normal user forgets to reset a protect after once releasing it by inputting the password to read out a program such that it becomes possible for anybody who comes later to read out the program.